I have also raised this issue. I played a game as France because I have never understood why they folded so quickly when they had the biggest army in Europe.
In the game, they only have a small force to counter the Germans. It's been explained to me that this has been done to accurately reflect just how rubbish they were. It makes the fall of France historically accurate because there is nothing France can do.
A good what-if scenario would be to use the editor to give the French additional units you think they should have. However, do you really want the French to defy history? A victorious France would force everyone on the continent to buy bread that goes stale after 2 hours, eat snails/frogs/stop washing and use perfume.
I think the games modelling of France is flawed, but it works for humanity in the long run.
I wouldn't say the French military were "rubbish" in 1940. They were comparable to the Germans in terms of manpower; and they actually had more tanks - and better equipment in some respects. Germans obviously had better leadership, doctrine, and morale. But the Germans left some 20 divisions in the Western front when they went into Poland, and it strains credulity to believe that the French would not have done serious damage with five times the personnel that the Germans had defending. Of course, I guess Gamelin could not have gone all-out with the French civilian leadership wanting to avoid - or at least postpone - a full-scale war; but what if the French were led by someone like Churchill, someone who recognized the true threat that Hitler posed?
The game-play repercussions of approximating the real French strength in 1939 are serious, I agree, and I guess that's the crux of the problem.
The GAME dictates the french are weak. You cannot succeed as France. The best you can do is add a month or two before surrendering.